This invention relates to application of a sliding mode control process to an engine air-fuel ratio control.
U.S. Pat. No. 6,266,605 discloses an application of a sliding mode control process to the feedback control of the air-fuel ratio of an air-fuel mixture supplied to an internal combustion engine.
In this prior art technique, an air-fuel ratio sensor is provided upstream and downstream of a catalyst in an exhaust passage, respectively, and calculates an output target value of the upstream air-fuel ratio sensor for which the output of the down-stream air-fuel ratio sensor is a predetermined value by a sliding mode control process.
The fuel injection amount is then feedback controlled by the sliding mode control process so that the output of the upstream air-fuel ratio sensor coincides with the target output value.
The dynamic characteristic of the down-stream air-fuel ratio sensor output relative to the upstream air-fuel ratio sensor output is expressed as a secondary discrete model for feedback control which makes the output of the down-stream air-fuel ratio sensor coincide with a predetermined value. The output of the down-stream air-fuel ratio sensor after a dead time has elapsed is estimated using the model, based on the output of the upstream air-fuel ratio sensor, and the dead time between the outputs of the two sensors is compensated.
The compensation of the dead time in the prior art technique is limited between the output of the two sensors. No correction is made for the dead time from fuel injection to the output of the upstream air-fuel ratio sensor. Also, the classic proportional integral differential (PID) controller or a self-tuning regulator is applied to the feedback control of the fuel injection amount.
For this reason, if the feedback gain is increased in the low air flowrate region near idle operation of the engine, where there is virtually a large dead time, the air-fuel ratio will tend to oscillate.
It is therefore an object of this invention to stabilize an air-fuel ratio control under sliding mode control with a large feedback gain.
In order to achieve the above object, this invention provides an air-fuel ratio control device for an engine with a combustion chamber for burning a mixture of air and fuel. The device comprises a fuel injector which injects fuel in the engine to generate the mixture, a sensor which detects an air-fuel ratio of the mixture from a composition of exhaust gas of the engine, and a programmable controller.
The programmable controller is programmed to calculate a dead time which represents a lag between the variation of air-fuel ratio of the mixture in the combustion chamber and the variation of air-fuel ratio which is detected by the sensor, calculates an estimated state quantity by applying a dead time compensation according to Smith method and a disturbance compensation to the air-fuel ratio detected by the sensor, calculate an air-fuel ratio feedback correction amount based on the estimated state quantity and a predetermined target air-fuel ratio through a sliding mode control process, and control a fuel injection amount of the fuel injector based on the air-fuel ratio feedback correction amount.
This invention also provides an air-fuel ratio control method for an engine with a combustion chamber for burning a mixture of air and fuel, and a fuel injector which injects fuel in the engine to generate the mixture. The method comprises detecting an air-fuel ratio of the mixture from a composition of exhaust gas of the engine, calculating a dead time which represents a lag between the variation of air-fuel ratio of the mixture in the combustion chamber and the variation of a detected air-fuel ratio, calculating an estimated state quantity by applying a dead time compensation according to Smith method and a disturbance compensation to the detected air-fuel ratio, calculating an air-fuel ratio feedback correction amount based on the estimated state quantity and a predetermined target air-fuel ratio through a sliding mode control process, and controlling a fuel injection amount of the fuel injector based on the air-fuel ratio feedback correction amount.
The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings.